This invention relates to a positioning controlling device which improves the follow-up accuracy and so forth of a positioning servo system.
In a numerically controlled machine tool, a position of a tool and a rotational position of a work are controlled by a servo system which employs servomotors. When a work is machined into a non-circular shape, target positions of such movable members are instructed momentarily. Instruction values of the target positions which vary with respect to time provide input functions of the servo system. Meanwhile, actual positions of the movable members which vary with respect to time in response to such instruction values provide response functions of the servo system.
Actually, since the response function is not equal to the input function, even if the input function is set in accordance with an ideal machining shape of a work, an actual finish shape is not equal to a preset machining shape. Therefore, such measures are taken that a transfer function of the servo system is measured and, taking such transfer function into consideration, the input function is calculated reversely so that the response function may correspond to a machining shape.
The transfer function of the positioning servo system is measured in such a manner as illustrated in FIG. 6. A white noise is inputted to the positioning servo system, and a spectrum analysis of a response function thus measured is carried out to measure the transfer function. Or else, a sine function of a certain frequency having a fixed magnitude is inputted to the positioning servo system, and a magnitude and a phase of a response function then are measured. Such measurement is carried out while varying the frequency of the sine function to find out the transfer function. A servo analyzer is employed as a device for carrying out such spectrum analysis to find out such a transfer function of a servo system as described above.
However, a positioning servo system has, in a mechanical system, a non-linearity arising from sliding friction, and in an electric system, a non-linearity originating in an analog element or the like employed in a servo amplifier, a digital to analog converter or the like. Accordingly, even if a sine function of a fixed frequency is used as an input function in a servo system, a magnitude and a phase of a response function will vary in accordance with a magnitude of the input function. In other words, a transfer function of a servo system varies depending upon levels of frequency components of an input function.
Accordingly, a transfer function of a servo system wherein individual frequency components are measured with an input function fixed to a predetermined level as in a prior art arrangement does not really represent a transfer function of the servo system with respect to an input function upon working when the level is different for the individual frequency components. Consequently, even if the input function is corrected with the transfer function, the response function does not ideally correspond to a machining shape of a work. Accordingly, the prior art technique has a problem that a sufficiently high degree of machining accuracy cannot be attained.